1. Field of the Invention
The invention is directed to an image sensor unit, and more particularly to a CMOS image sensor unit and a method for fabricating the same.
2. Description of Related Art
A typical CMOS image sensor unit includes a photodiode (PD), a transfer gate, a reset gate, a source follower gate, and a floating drain region (FD) between the transfer gate and the reset gate. The floating drain region is electrically connected with the source follower gate through a metal interconnect. During the operation, the charges generated from the photodiode having received a light signal flows to the floating drain region through the channel controlled by the transfer gate to influence the magnitude of the current of the channel controlled by the source follower gate which is electrically connected with the floating drain region, such that the intensity of the light signal can be derived.
However, for the capacitance (CPD) of the entire photodiode region is larger than that (CFD) of the floating drain region, charges generated in the photodiode are difficult to totally transfer to the floating drain region during the period of switching on the transfer transistor. Thus, after the transfer transistor is switched off, some charges remain in the photodiode and lead to an image lag. Therefore, the dynamic range of image sensing is limited.
In a solution in the prior art, a metal/insulator/metal capacitor coupled to the floating drain region is added to increase the capacitance of the floating drain region. However, if doing so, an additional area is occupied reducing the fill factor. Additionally, in the conventional structure, current signal transmission between the floating drain region and the source follower gate passes through the hetero junction between the metal and the silicon in the interconnect. The leakage existing in the hetero junction easily causes a dark current during the image sensing, so that noise is formed during signal sensing.